In a typical thermal printer, a web-type dye-carrier containing a series of spaced frames of different colored heat transferrable dye is spooled onto a carrier supply spool. The carrier is paid out from the supply spool and rewound on a take-up spool. The carrier moves through a nip formed between a thermal print head and a dye-absorbing receiver sheet. In one particular arrangement, the receiver sheet is clamped to a rotatable drum. The receiver sheet may, for example, be coated paper and the print head is formed of a plurality of heating elements. When heat is selectively applied from the heating elements to the dye-carrier, dye is transferred to the receiver sheet. As shown in commonly assigned U.S. Pat. No. 4,815,870, at the beginning of a print cycle, the receiver sheet is clamped to the drum. After being clamped to the drum, the receiver is advanced under the print head. The heating elements of the print head are energized to form a dye image. The drum makes several revolutions as different colored dye images are applied into the receiver. In this way, a final, full-colored image is produced. After this full-colored image is produced, the direction of the drum is reversed, and when a position is reached the clamp is opened and the receiver sheet is ejected from the thermal printer.
In certain printers, particularly in those where the image is of great length, it is preferable to use a small diameter drum which as used herein means the drum diameter is selected so that its circumference is less than the length of the receiver. In cases where the image is, for instance, an A size image (8.5 inches by 11 inches), a corresponding drum would be over 3.5 inches in diameter. Such a print drum represents significant costs and volume in design of a printer. In addition, such a drum increases the load and precision requirements on the drive system. The reach at the head (distance between the nip and the position where the head is pivotably mounted) also increases when large drums are used. This length increases the cost of the head. The reduced size of the head in small drum printers represents a significant cost savings. In these small drum printers, the platen may complete several rotations before a color plane is deposited. After the dye plane is deposited, it is necessary to re-position the paper so that the first line of the image is back under the thermal head.
In most of these small drum printers, the drum is rotated in a first direction as a colored dye image is printed in the receiver. After this image is printed, the direction of the drum is reversed and the sheet is returned to the start position. Successive dye layers are deposited until a complete image is formed. Drums used in these printers are covered with an elastomeric surface for two reasons; to provide media compliance to the thermal head, and to provide a high friction surface between the receiver and drum so as to allow for accurate metering of the receiver during printing. Because this surface is an elastomer, there is a low resistance to material flow and twist both during printing and rewind. Depending on the properties of the receiver and elastomer, typical registration in certain printers will be about 0.010", worse case. This misregistration is also affected by the type of image printed, and even environmental and aging factors.